DOUBLE LAYER CAPACITANCE ON PLATINUM IN 1M H2S04 FROM REVERSIBLE HYDROGEN POTENTIAL TO OXYGEN FORMATION REGION

Improved design and electronics which gave maximum oscilloscope screen sensitivities of 5 mV and 5 nsec/div permitted measurements with single, relatively high intensity galvanostatic pulses of short duration so that the net perturbation of the system from equilibrium or steady state was small. This arrangement gave accurate separation of double layer, faradaic, and adsorption/ desorption processes at times less than 200 nsec. The double layer capacitance at a clean, bright Pt electrode in 1M H2SO4 was measured from the reversible H2/H+ equilibrium potential to 1.2V vs. N.H.E. The double layer capacitance of the normal equilibrium hydrogen electrode on Pt was found to be 17 ± 1 µF/cm2 (all measurements were on a true area basis). The double layer capacitance increases to 30 µF/cm2 at 0.15V and uniformly decreases to 17 µF/cm2 at 0.7V. Above 0.9V, the formation of highly polarized oxygen atoms on the Pt causes the capacitance to increase. The exchange current density for the H⇄H+ + e reaction was found to be 0.44 A/cm2. No “ideally” polarized potential region was found. © 1969, The Electrochemical Society, Inc. All rights reserved.